Lithium-ion secondary batteries that include a positive electrode; a negative electrode in which carbon particles are applied as a negative electrode active material layer to a surface of a negative electrode current collector made from a copper foil whose two sides are flat, dried, and then pressed; and a non-aqueous electrolyte, are currently used in mobile phones and notebook personal computers, and the like. So-called “unprocessed copper foil” that is produced by electrolysis and that has been rust-proofed is used in the negative electrode of the lithium-ion secondary battery.
As disclosed in Patent Document 1 (Japanese Patent No. 3742144), by using a copper foil in which the roughness of the matt surface is reduced thereby reducing the difference in the surface roughness of a glossy surface (S surface) and a matt surface (M surface) and (both surfaces of the copper foil) as the copper foil used as the negative electrode current collector for a lithium-ion secondary battery, the reduction of the charge-discharge efficiency of the secondary battery is suppressed.
Electrolytic copper foil in which the roughness of the matt surface has been reduced as described above to reduce the difference in surface roughness of a first surface (glossy surface, S surface) and a second surface (matt surface, M surface) is produced by appropriately selecting various water soluble polymer substances, various surfactants, various organic sulfur compounds, chloride ions, and the like, and adding them to a copper sulfate-sulfuric acid electrolytic solution.
Patent Document 1 (Japanese Patent No. 3742144) discloses a negative electrode current collector using electrolytic copper foil produced by adding a compound having a mercapto group, chloride ion, low molecular weight glue having a molecular weight of 10000 or less, and polymer polysaccharide to copper sulfate-sulfuric acid electrolytic solution.
Carbon particles were applied to the surface of the electrolytic copper foil produced by the above manufacturing method, dried, and then pressed to produce the negative electrode.
This electrolytic copper foil has a tensile strength of from 300 to 350 N/mm2, and also has an appropriate elongation, so it is a suitable material for use as the copper foil for a negative electrode having carbon particles as the active material.
Patent Document 2 (Japanese Patent No. 3850155) discloses an electrolytic copper foil produced from copper sulfate-sulfuric acid electrolytic solution.
In recent years, lithium-ion secondary batteries using aluminum, silicon, tin, and the like that alloys electrochemically with lithium during charging as the negative electrode active material have been proposed with the objective of increasing the capacity of the lithium-ion secondary battery (see Patent Document 3 (Japanese Unexamined Patent Application Publication No. H10-255768)).
In order to increase the capacity, the electrode (negative electrode) for the lithium-ion secondary battery is formed by laminating silicon as a noncrystalline silicon thin-film or a microcrystalline silicon thin film on the current collector made of copper foil or the like, using the CVD method or the sputtering method. It has been found that the active material thin-film layer formed by these methods adheres to the current collector, so good charge-discharge cycle properties are exhibited (see Patent Document 4 (Japanese Unexamined Patent Application Publication No. 2002-083594)).
Recently, a method of forming has been developed in which silicon powder or silicon compound is made into a slurry using an organic solvent and an imide binder, applied to the copper foil, dried, and pressed (see Patent Document 5 (Japanese Unexamined Patent Application Publication No. 2007-227328)).
However, in this type of electrode for a lithium-ion secondary battery, for example, during charging, the silicon active material swells in volume by a factor of about 4 due to storage of lithium ions. In addition, it contracts during discharging by releasing lithium ions.
As a result of this swelling and contraction in the volume of the active material layer associated with charging and discharging, a phenomenon in which the active material becomes comminuted to a fine powder and separates from the current collector has been found.
Also, as a result of the adhesion of the active material layer to the current collector, when the volume of the active material layer swells and contracts due to repeated charging and discharging, a large stress acts on the current collector, creases are generated on the current collector, and, in addition, after repeated charging and discharging many times, there is the problem that the foil that constitutes the current collector breaks.
The following is a more detailed description of this problem.
When the active material is reduced to fine powder and separates from the current collector, a reduction in the battery charge-discharge cycle efficiency occurs.
When the current collector is deformed by creasing, or the like, the problem that a short-circuit can easily occur between the positive electrode and the negative electrode occurs.
When the current collector breaks, the problem that it is not possible to maintain the battery performance stable over a long period of time occurs.
In response to such a problem, the inventors of the present invention proposed adopting copper foil with high tensile strength and large breaking elongation (see Patent Document 6 (WO2010/110205)). For example, it was proposed that a lithium-ion secondary battery be configured by adopting electrolytic copper foil with tensile strength of 400 N/mm2 or more, an elongation of from 4.5% to 13%, and a surface roughness Ra of from 0.01 to 1 μm.
Patent Document 6 (WO2010/110205) describes an example that uses a silicon active material in the active material layer.
However, when silicon active material is used, heat may have to be applied to the copper foil that is the current collector, for example, if a silicon active material is used and if a polyimide binder is used, the drying and curing temperature may be from 200° C. to 400° C.
In this case, it has been found that the performance of the electrolytic copper foil as a current collector indicated in Patent Document 6 (WO2010/110205) cannot be sufficiently exhibited.